As the world's population expands and its economy increases, the increase in the atmospheric concentrations of carbon dioxide is warming the earth causing climate changes. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. Experts say atmospheric levels of carbon dioxide may be double that of the pre-industrial era by the end of the next century, but they also say the levels would be much higher except for a trend toward lower-carbon fuels that has been going on for more than 100 years. Furthermore, fossil fuels cause pollution and are a causative factor in the strategic military struggles between nations. Furthermore, fluctuating energy costs are a source of economic instability worldwide.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The present invention is another product which is essential to shortening that period to a matter of years. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But, with the problems of storage and infrastructure solved (see U.S. application Ser. No. 09/444,810, entitled “A Hydrogen-based Ecosystem” filed on Nov. 22, 1999 for Ovshinsky, et al., which is herein incorporated by reference and U.S. patent application Ser. No. 09/435,497, entitled “High Storage Capacity Alloys Enabling a Hydrogen-based Ecosystem”, filed on Nov. 6, 1999 for Ovshinsky et al., which is herein incorporated by reference), hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
Hydrogen is the “ultimate fuel.” In fact, it is considered to be “THE” fuel for the future. Hydrogen is the most plentiful element in the universe (over 95%). Hydrogen can provide an inexhaustible, clean source of energy for our planet which can be produced by various processes. Utilizing the inventions of subject assignee, the hydrogen can be stored and transported in solid state form in trucks, trains, boats, barges, etc. (see the '810 and '497 applications).
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. While other fuels are being tried, hydrogen is the primary fuel of choice because of its simplicity and the nature of the end product. Researchers have been actively studying fuel cells to utilize the fuel cell's potential high energy-generation efficiency. The base unit of the fuel cell is a cell having an oxygen electrode, a hydrogen electrode, and an appropriate electrolyte. Fuel cells have many potential applications such as supplying power for transportation vehicles, replacing steam turbines and power supply applications of all sorts. Despite their seeming simplicity, many problems have prevented the widespread usage of fuel cells.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with fuel (reactants) from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are continuously removed from the system, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or other energy generating systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation reaction and the oxygen electrode for oxygen reduction reaction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a specially designed porous hydrogen electrode and oxygen electrode and brought into surface contact with the electrolyte. The particular materials utilized for the hydrogen electrode and oxygen electrode are important since they must act as efficient catalysts for the reactions taking place.
In a hydrogen-oxygen alkaline fuel cell, the reaction at the hydrogen electrode occurs between hydrogen fuel and hydroxyl ions (OH−) present in the electrolyte, which react to form water and release electrons as shown in the overall reaction:H2+2OH−->2H2O+2e−At the oxygen electrode, oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−->4OH−The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
The catalyst in the hydrogen electrode of the alkaline fuel cell has to not only split molecular hydrogen to atomic hydrogen, but also oxidize the atomic hydrogen to release electrons. The overall reaction can be seen as (where M is the catalyst):M+H2->2 MH->M+2H++2e−Thus the hydrogen electrode catalyst must efficiently dissociate molecular hydrogen into atomic hydrogen. Using conventional hydrogen electrode materials/catalysts, the dissociated hydrogen atoms remain transitional and the hydrogen atoms can easily recombine to form molecular hydrogen if they are not used directly in the oxidation reaction as a number of conditions can prevent a quick oxidative response.
In a zinc-air fuel cell, a type of metal-air fuel cell, the reaction at the anode occurs between the zinc contained in the anode and hydroxyl ions (OH−) present in the electrolyte, which react to release electrons:Zn->Zn+2+2e−Zn+2+2(OH−)->Zn(OH)2Zn(OH)2+2(OH)—->ZnO2−2+2H2OAt the cathode, oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−->4OH−.The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen and oxygen electrodes.
Fuel cells, when used to power vehicles, are often used with an auxiliary battery pack because of the general inability of fuel cells to provide power instantly upon start-up or provide increased bursts of power for sudden acceleration. Such vehicles are generally termed hybrid electric vehicles (HEV). The auxiliary battery supplements the fuel cell power output during conditions requiring high power output, such as during start-up or sudden acceleration. The auxiliary battery may be used for the absorption of regenerative braking power.
Hybrid systems have been divided into two broad categories, namely series and parallel systems, which may include an internal combustion engine or fuel cell supplied. The internal combustion engine is supplied with a combustible fuel while the fuel cell is supplied with hydrogen or a fuel capable of providing hydrogen atoms needed for power generation. In a typical series hybrid vehicle system utilizing an internal combustion engine, as shown in FIG. 1, a battery 1 powers an electric propulsion motor 2 which is used to propel a vehicle 3 and an internal combustion engine 4a is used to recharge the battery 1. In a typical series hybrid vehicle system utilizing a fuel cell, as shown in FIG. 2, a battery 1 powers an electric propulsion motor 2 which is used to propel a vehicle 3 and the fuel cell 4b is used to recharge the battery. In a typical parallel hybrid vehicle system utilizing an internal combustion engine, as shown in FIG. 3, both the internal combustion engine 4a and the battery 1 in conjunction with an electric motor 2 can be used, either separately or together, to propel a vehicle 3. The internal combustion engine 4a may also be used to recharge the battery 1. In a typical parallel hybrid vehicle system utilizing a fuel cell, as shown in FIG. 4, both the battery 1 and the fuel cell 4b are used to power an electric motor 2 which propels the vehicle 3. The fuel cell 4b may also be used to recharge the battery 1. In the types of vehicles utilizing a parallel hybrid vehicle system, the battery is usually used only in short bursts to provide increased power upon demand after which the battery is recharged using the internal combustion engine or via feedback from a regenerative braking process. Shown in FIG. 5 and FIG. 6, are further detailed schematics of hybrid systems utilizing fuel cells. The schematic shown in FIG. 5, shows an example of a parallel hybrid vehicle system including a battery and a fuel cell 4b, both of which supply power to a power logic control 6, which powers an electric motor 2, and a hydrogen storage unit 5, which provides hydrogen to the fuel cell. Power is also supplied from the power logic control 6 to a hotel load 7, which provides power to other vehicle components. In this design, both the fuel cell 4b and the battery 1 power the electric motor 2, while power from the fuel cell 4b may be used to recharge the battery 1. Power may also be generated through regenerative braking and used to recharge the battery 1. In this system, the battery is used for on demand surge power and the fuel cell is used for on demand load leveling power. Since the battery is used only for on demand surge power, the battery requires only a small amount of energy storage. The schematic in FIG. 6 shows range extender series hybrid system design including a battery 1 and a fuel cell 4b, both of which supply power to a power logic control 6, which provides power to an electric motor 2, and a hydrogen storage unit 5, which provides hydrogen to the fuel cell. Power is also supplied from the power logic control 6 to a hotel load 7, which provides power to other vehicle components. In this system, the battery 1 is essentially powering the electric motor 2 while the power from the fuel cell 4b is used to recharge the battery 1. This system requires a large amount of battery energy storage with the range of the vehicle being related to the amount of hydrogen stored onboard the vehicle.
There are further variations within these two broad categories. One variation is made between systems which are “charge depleting” in the one case and “charge sustaining” in another case. In the charge depleting system, the battery charge is gradually depleted during use of the system and the battery thus has to be recharged periodically from an external power source, such as by means of connection to public utility power. In the charge sustaining system, the battery is recharged during use in the vehicle, through regenerative braking and also by means of electric power supplied from the a generator powered by the internal combustion engine so that the charge of the battery is maintained during operation.
There are many different types of systems that fall within the categories of “charge depleting” and “charge sustaining” and there are thus a number of variations within the foregoing examples which have been simplified for purposes of a general explanation of the different types. However, it is to be noted in general that systems which are of the “charge depleting” type typically require a battery which has a higher charge capacity (and thus a higher specific energy) than those which are of the “charge sustaining” type if a commercially acceptable driving range (miles between recharge) is to be attained in operation.
A key enabling technology for HEVs is having an energy storage system having a high energy density while at the same time being capable of providing very high power. Such a system allows for recapture of energy from braking currents at very high efficiency while enabling the design of a smaller battery pack.
A typical auxiliary battery pack as used in HEV applications is a nickel metal hydride battery pack. In general, nickel-metal hydride (Ni-MH) cells utilize a negative electrode comprising a metal hydride active material that is capable of the reversible electrochemical storage of hydrogen. Examples of metal hydride materials are provided in U.S. Pat. Nos. 4,551,400, 4,728,586, and 5,536,591 the disclosures of which are incorporated by reference herein. The positive electrode of the nickel-metal hydride cell comprises a nickel hydroxide active material. The negative and positive electrodes are spaced apart in the alkaline electrolyte.
Upon application of an electrical current across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the absorption of hydrogen formed by electrochemical water discharge reaction and the electrochemical generation of hydroxyl ions: The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The charging process for a nickel hydroxide positive electrode in an alkaline electrochemical cell is governed by the following reaction: 
During charge, the nickel hydroxide is oxidized to form nickel oxyhydroxide and during discharge, the nickel oxyhydroxide is reduced to form nickel hydroxide as shown by the following reaction: 
While the inclusion of an auxiliary battery pack working in conjunction with a fuel cell has many advantages for powering vehicles, such systems still have disadvantages upon implementation in a vehicle. The disadvantages of including a battery along with the fuel cell may include increased weight, increased space, extra terminals, inter cell connects, increased cost, increased maintenance, etc. Improvements in these areas will help fuel cells to become the standard source of power for vehicles and many other applications.